1. Field of the Invention
The present invention relates to a laser, such as a distributed feedback (DFB) semiconductor laser capable of switching a polarization mode of its output light between two polarization modes (typically, transverse electric (TE) mode and transverse electric (TM) mode) depending on its driven condition, and relates to a method of driving that laser and to an apparatus or system including the laser.
2. Related Background Art
Conventionally, Japanese Patent Application Laid-Open No. 7(1995)-162088, for example, discloses a polarization-mode switchable DFB semiconductor laser with plural electrodes in which the relation between a wavelength dispersion of a gain created by its active layer and a Bragg wavelength determined from a pitch of its diffraction grating and so forth, is controlled.
FIG. 1 illustrates a conventional structure of such a type. FIG. 1 is a cross-sectional view taken along a laser resonance (cavity-axial) direction of the DFB semiconductor laser. The structure includes a lower clad layer 1010, an active layer 1011, a light guide layer 1012, an upper clad layer 1013, and a contact layer 1014 which are laid down over a substrate 1009, in this order. A diffraction grating 1020 is formed at the interface between the light guide layer 1012 and the upper clad layer 1013. The contact layer 1014 is divided into two portions along the resonance direction. Electrodes 1002 and 1003 are respectively deposited on the two portions of the contact layer 1014, and an electrode 1008 is formed on the bottom surface of the substrate 1009. Currents can be independently injected into two active regions under the electrodes 1002 and 1003, which are electrically independent from each other along the laser resonance direction. An antireflection layer 1004 is provided on an end facet of the laser, and a separating groove 1015 is formed between the two active regions.
In the conventional structure, the active layer 1011 is formed of a quantum well structure of AlGaAs and GaAs. The Bragg wavelength of the grating 1020 is set at a value shorter than a peak wavelength of the gain spectrum for the TE mode. Thus, a polarization mode contention condition can be created between the TE mode and the TM mode. A ratio between the currents injected into the two active regions can be controlled, so that the polarization mode of its oscillated output light can be switched between the TE mode and the TM mode.
Further, Japanese Patent Application Laid-Open No. 2(1990)-159781 discloses a three-electrode DFB semiconductor laser with a xcex/4 phase shift section in its diffraction grating, which can switch the polarization mode of its output light between the TE mode and the TM mode. The semiconductor laser includes a structure in which currents can be independently injected into a region with the xcex/4 phase shift and a region without it. The xcex/4 phase shift section is formed in a central portion, and currents can be independently injected into the central portion and two remaining portions on both opposite sides thereof. When the current injected into the central region with the xcex/4 phase shift is changed under a uniform current injection condition, the oscillation polarization mode can be switched between the TE mode and the TM mode.
Furthermore, Japanese Patent Application Laid-Open No. 8(1996)-172234 discloses a polarization-mode switchable semiconductor laser with a phase controlling or adjusting region lacking a diffraction grating and an active layer, in which a difference of about xcfx80 is generated between phase changes for the TE mode and the TM mode in the phase controlling region. The polarization dependency of the amount of the phase change is thus set such that the oscillation polarization mode can be stably switched.
Each of the above structures effects the polarization switching when its light circulation phase is changed. Further, each has a structure satisfying the following condition: While the mode is being changed to a mode whose circulation phase differs from the present mode by 2xcfx80, for the same polarization mode (for example, TE mode), the circulation phase of light in the other polarization mode (for example, TM mode) comes to satisfy the resonance condition and the pumping amount comes to reach its threshold gain. For this purpose, a strained quantum well is used in the conventional structure, for example. This approach can also be used in the present invention.
However, in the conventional polarization-mode switchable DFB laser with the phase adjusting region wherein a diffraction grating region and a phase adjusting region are arranged serially, a sufficient effect of light adjusted in the phase adjusting region often cannot be obtained when light is only weakly returned from the phase adjusting region, or when the coupling coefficient of the diffraction grating is large and thus its reflection factor is high.
It is an object of the present invention to provide a laser, such as a distributed feedback semiconductor laser, which is constructed such that light influenced by its phase controlling or adjusting region can readily influence a region adjacent to the phase controlling region effectively, and to a driving method for driving the laser, a light transmitter using the laser, and an optical transmission system or method using the laser.
The object of the present invention is achieved by the following lasers, driving methods, transmitters and optical communication systems or methods.
A laser of this invention includes a first region with a first waveguide having a first diffraction grating, and a second region wave waveguide having a second diffraction grating, and a phase controlling region with a third waveguide and a phase control unit for controlling an effective refractive index of the third waveguide. The phase controlling region, the first region and the second region are serially coupled along a light propagation direction in this order, and are constructed such that light to the first region from the phase controlling region is enlarged relatively to light to the phase controlling region from the first region, or constructed such that a coupling coeffecient of the first diffraction grating in the first region adjacent to the phase controlling region is smaller than a coupling coefficient of the second diffraction grating in the second region.
The effect or performance of the phase controlling region is enhanced, and hence the modulation of polarization-mode or wavelength can be effectively achieved stably. Further, the action of the phase controlling region can be effectively employed by making the coupling coefficient in the first region adjacent to the phase controlling region smaller than the coupling coefficient in the second region away from the phase controlling region, though the coupling coefficient of the region other than the phase controlling region is not decreased uniformly.
More specifically, the following structures may be adopted based on the above fundamental structures.
A coupling coefficient of the first diffraction grating in the first region adjacent to the phase controlling region may be set smaller than a coupling coefficient of the second diffraction grating in the second region. In this structure, the grating region with the smaller coupling coefficient allows a large change of light at a wavelength at which phases of light travelling from the phase controlling region and light reflected in the grating region with the smaller coupling coefficient coincide with each other, by the control of the phase of light in the phase controlling region. Accordingly, a resonance wavelength balancing in three regions of the regions with large and small coupling coefficients and the phase controlling region can be largely changed by the change of the index in the phase controlling region due to the control of current injection, voltage application or the like. As a result, the effect of the phase controlling region can be enhanced, and hence the modulation of polarization mode or wavelength can be effectively and stably achieved. The feature of this structure is that the grating region is devised such that light influenced by the phase controlling region can readily influence the grating region adjacent to the phase controlling region.
Further, the following specific structures may be used as a structure for making light to the first region from the phase controlling region larger than light to the phase controlling region from the first region.
The first region includes a first control unit for pumping the first region, and the phase control unit and the first control unit are capable of independently controlling the phase controlling region and the first region, respectively. In this structure, a feedback function of the region adjacent to the phase controlling region is weakened relatively to that of the other region, so that the influence of light from the phase controlling region can be readily increased. As a result, the effect of the phase controlling region can be enhanced, and hence the modulation of polarization mode or wavelength can be effectively and stably achieved. The feature of the structure is that a feedback to the region adjacent to the phase controlling region due to the gain can be decreased.
The laser may be typically constructed as a distributed feedback semiconductor laser. In this case, the phase controlling region may include a cleaved end facet. A reflective layer may be provided on the cleaved end facet. In this structure, the end facet with a large reflection factor can be used, different from a case where the phase controlling region is arranged near a central portion of the DFB laser, and thus the effect of the phase controlling region can be further increased.
According to still another aspect of the present invention, there is provided a method for driving a laser in which a current injected into or a reverse voltage applied to the phase controlling region is changed to change at least one of a polarization mode and a waveguide of light output from the laser.
According to still another aspect of the present invention, there is provided a light transmitter which includes the above laser, a control unit for controlling light output from the laser in accordance with a transmission signal, and a mode selector for selecting a component of a desired mode from the light output from the laser. The mode selector may be a polarization-mode selector or a wavelength selector.
According to still another aspect of the present invention, there is provided an optical communication system for communicating over a light transmission line that transmits a signal from a transmitter side to a receiver side, in which light of a signal from the above transmitter is transmitted through the light transmission line, and a receiver receives and detects an intensity-modulated signal transmitted from the transmitter through the light transmission line. The system may be a wavelength division multiplexing optical communication system, in which a light transmission line transmits a plurality of intensity-modulated signals at a plurality of wavelengths generated by a plurality of the above transmitters, and a wavelength selector, such as a tunable band-pass filter, selects the intensity-modulated signal at a desired wavelength to be detected on a receiver side.
These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings.